1. Technical Field of the Invention
The present invention relates to identifying a source of interference in a telecommunications network, and particularly identifying primary sources of uplink and downlink interference in a mobile cellular telecommunications network.
2. Background of the Invention
In cellular telephone communications systems (henceforth referred to as xe2x80x9ccellular systemsxe2x80x9d, or simply xe2x80x9csystemsxe2x80x9d), it is important to maximize traffic-handling capacity, because the demand for such capacity continues to increase. One factor that affects capacity is the way the totality of available communication channels are allocated for use by particular cells within the system. The use of the same channel by two or more cells that are in relatively close proximity to one another can cause each cell to experience too much co-channel interference, and should therefore be avoided.
One solution for avoiding co-channel interference is to have each cell operate on a dedicated group of channels that are not used by any other cell in the system. Although this strategy successfully avoids the occurrence of co-channel interference, it limits the system""s traffic-handling capacity to the number of channels that the system is permitted to use.
In order to increase the system""s traffic-handling capacity, it is possible to devise a reuse plan whereby any one channel may be concurrently used by two or more cells. Co-channel interference is limited by ensuring that the one channel is allocated to cells that are geographically located far enough apart (referred to as the xe2x80x9creuse distancexe2x80x9d) so as not to severely interfere with one another. The appropriate distance for limiting interference will depend upon factors that affect the carrier to interference ratio (C/I) on that particular channel in each cell.
Despite a carefully devised reuse plan, co-channel interference may nonetheless occur. Co-channel interference may be on the downlink in which a mobile station in one cell receives an interfering signal transmitted by a base station in another cell towards a mobile station therein. Co-channel interference may also occur on the uplink in which a base station in one cell receives an interfering signal from a mobile station that is communicating with a signal to a base station in another cell. Because existing reuse plans reuse transmission frequencies a number times within a cellular network, co-channel interference experienced within a first cell, the xe2x80x9cdisturbedxe2x80x9d cell, may be attributed to one or more other cells which use the same set of frequencies. There exists a need to identify those cells within a mobile cellular network which contribute to interference within a disturbed cell.
A number of methods are used to identify the contributor of interference experienced within a disturbed cell from other candidate cells in the cellular network. One technique is to turn on one possible contributor of interference at a time and drive test the disturbed cell with a test receiver. Drive testing, however, is costly with respect to both time and resource usage. In addition, due to its intrusive nature, this technique is only performed during low traffic periods, i.e., at night.
A second prior technique requires changing the voice channel assignment algorithms in an attempt to de-emphasize the influence of at least some of the candidate cells, thereby reducing the number of interfering cell candidates. This technique, however, disadvantageously requires a change in the network configuration and thus may degrade the quality of service within cells associated with the change in the voice channel assignment algorithms.
Other known techniques require extensive data collection from the disturbed cell and the candidate cells and extensive analysis of the collected data. These techniques substantially increase the amount of processing within the mobile network, which itself operates at a significantly high capacity in performing conventional telecommunication services. Noting the importance of collecting data during times of high traffic, these prior techniques have been found to overly burden the mobile telecommunications network.
Based upon the foregoing, there is a need for efficiently and accurately identifying primary contributors of interference occurring within a cell of a mobile cellular telecommunications network.
The present invention overcomes the shortcomings in prior systems and thereby satisfies a significant need for a method and system for identifying one or more primary contributors of interference within a disturbed cell in a mobile cellular telecommunications network. The identification of a primary contributor and/or cause of interference within a disturbed cell provides network planners and/or system operators an opportunity to intelligently modify the parameters by which wireless communication within one or more cells is performed.
According to a preferred embodiment of the present invention, the system includes devices for collecting data within the disturbed cell and a number of candidate cells, each of which may potentially be the cause of interference within the disturbed cell. Depending upon whether interference is experienced in uplink or downlink communication, the data collecting devices may be associated with the radio base station of the disturbed cell and each candidate cell, or with mobile stations communicating with the disturbed cell and each candidate cell. The data collection devices are adapted to measure the interference level within the disturbed cell and the level of traffic within each of the candidate cells. The interference level and traffic level measurements are collected within each of a number of sampling periods that occur at preselected times over a predetermined period of time.
The system, according to the preferred embodiment of the present invention, utilizes various statistical processes to identify those candidate cells that cause interference within the disturbed cell. For each candidate cell, the system determines the correlation between traffic utilization within the candidate cell and the level of interference within the disturbed cell. This is based upon the premise that interference within the disturbed cell is linearly dependent upon the traffic level within a cell that is a primary contributor to the interference. Candidate cells having a low correlation between interference level within the disturbed cell and traffic level within the candidate cell are dismissed as being non-contributors of interference with respect to the disturbed cell.
Because candidate cells may have comparable traffic patterns throughout the term data is collected, a candidate cell which does not actually cause interference within the disturbed cell may nonetheless relatively closely yield a high correlation between the interference level within the disturbed cell and the traffic level in the candidate cell, thereby falsely identifying the candidate cell as an interfering cell. To address this false identification, the present invention determines the correlation between the traffic levels in the various candidate cells during the period of data collection. The preferred embodiment of the present invention additionally determines complicated correlation coefficients, such as the partial correlation coefficient between the traffic level in each candidate cell and the interference level in the disturbed cell with the effects of traffic levels in the other candidate cells being removed. The preferred embodiment of the present invention further determines the multiple correlation coefficient between the interference level in the disturbed cell and the traffic levels in all of the candidate cells. Having determined the partial correlation coefficients and the multiple correlation coefficient, the preferred embodiment of the present invention is capable of identifying the candidate cells which are the primary source of interference with communications within the disturbed cell as well as the extent of contribution of each primary source candidate cell to the total interference measured in the disturbed cell.
Accordingly, the system includes processing equipment for performing the above-mentioned statistical processes on the collected data. The processing equipment performs the correlation determinations off-line in order to minimize the load on the network""s resources.
The operation of the present system includes initially identifying the candidate cells, any of which may be a primary contributor of interference within the disturbed cell. Next, data is collected from each candidate cell as well as the disturbed cell during predetermined sampling intervals throughout a predetermined period of time. The collected data includes traffic level data for each candidate cell and interference level data for the disturbed cell. The collected data is then stored in a memory device accessible by the processing equipment.
Next, the processing equipment retrieves the collected data and initially prepares a covariance matrix from which is determined a simple correlation coefficient between the interference level of the disturbed cell to the traffic level in each candidate cell, and a simple correlation coefficient between the traffic levels in the candidate cells. From the simple correlation coefficients, the candidate cell which causes the most interference within the disturbed cell is obtained.
Further, the processing equipment determines for each candidate cell the partial correlation coefficient between the traffic level in the candidate cell and the interference level within the disturbed cell, with the effects of the traffic level in the other candidate cells being removed. The processing equipment then determines the multiple correlation coefficient between the interference level in the disturbed cell and the traffic levels in all of the candidate cells. Based upon the partial correlation coefficients and the multiple correlation coefficient, the processing equipment identifies the primary contributors of interfering candidate cells as well as the extent of interference contributed by each candidate cell.